The following abbreviations are used herein:
CoMPCoordinated Multi-Point (the abbreviation CoMP often alsomeans Coordinated Multi-Point transmission/reception, aswill be evident from the context)CQIChannel Quality IndicatorCSIChannel State Information - CSI includes PMI, RI, CQI (seebelow)DLDownlinkDMRSDemodulation Reference SignaleNodeBEvolved NodeB (i.e. evolved base station)JTJoint TransmissionMMSEMinimum Mean Squared ErrorPMIPrecoder Matrix IndicatorSINRSignal to Interference plus Noise RatioRIRank IndicatorTPTransmission PointUEUser Equipment
Also, the following mathematical notations are adopted herein:                |a| denotes the absolute value of a;        ∥a∥2=|a(1)|2+ . . . +|a(N)|2 (unless stated otherwise);        Ea denotes the expectation (or expected value) of a; and        For any matrix A, AH denotes the conjugate transpose of A, and tr(A) represents the operation of taking the trace of A.        
Joint Transmission Downlink Coordinated Multi-Point transmission/reception (JT-DL CoMP).
FIG. 1 schematically represents JT in a DL CoMP system. The system includes multiple TPs (these may be eNodeBs), each TP being equipped with multiple antennas, and multiple UEs where each UE is also equipped with multiple antennas. The multiple TPs transmit data to the multiple UEs on the same time-frequency. Generally, to minimise interferences between TPs and between UEs, the transmission is carried out with CoMP precoders which are generated based on (i.e. generated from the knowledge of) the channel state information (CSI).
Each UE feeds back CSI (which includes RI, PMI and CQI) to its serving TP via uplink, as illustrated in FIG. 2.
In CSI measurement, for each UE there are as many CSI configurations as there are TPs involved in JT-DL CoMP. FIGS. 3A and 3B show CSI measurement for a UE involved in JT-DL CoMP with two TPs. The CSI config#0 is for TP#0 (the serving TP) and the CSI config#1 is for TP#1 (the neighbouring TP).
System Description
A JT DL CoMP system having NTP TPs and NUE UEs may be described mathematically as set out below.
Let τn denote the number of antennas at the n-th TP. The total number of transmit antennas NTX used in DL CoMP transmission is:
      N    TX    =            ∑              n        =        1                    N        TP              ⁢                  τ        n            .      
Let NRX denote the number of receive antennas at each UE, and let Hin (size NRX×τn) denote the channel between the n-th TP and the i-th UE.
Then the DL CoMP channel of the i-th UE (size NRX×NTX) is:Hi=└Hi1,Hi2, . . . ,HiNTP┘,i=1, . . . ,NUE  Equation (1)
Let Vi (size NTX×RIi) denote the precoder for the i-th UE.
The received signal at the i-th UE (yi) is given by:
                                          y            i                    =                                                    H                i                            ⁢                                                ∑                                      j                    =                    1                                                        N                    UE                                                  ⁢                                                      V                    j                                    ⁢                                      x                    j                                                                        +                          n              i                                      ,                                  ⁢                  i          =          1                ,        …        ⁢                                  ,                  N          UE                                    Equation        ⁢                                  ⁢                  (          2          )                    where ni is additive Gaussian noise. Note that, from the DMRS, the i-th UE can find the effective channel HiVi to generate a decoder.
Precoding
Precoding is dependent on PMI which is part of the CSI. (Recall that CSI is fed back by a UE to its serving TP via uplink.) Let pin denote the PMI corresponding to Hin. Note that in a 2-stage PMI codebook system, pin is a pair PMI#1 and PMI#2.
According to the 3GPP standard (TS 36.211), the precoder Win (of size τn×RIi) associated with the reported PMI pin is used for precoding data to send from the n-th TP to the i-th UE. The total DL CoMP precoder is therefore given by:
                                          V            i                    =                      [                                                                                W                                          i                      ⁢                                                                                          ⁢                      1                                                                                                                                        W                                          i                      ⁢                                                                                          ⁢                      2                                                                                                                    ⋮                                                                                                  W                                          iN                      TP                                                                                            ]                          ,                                  ⁢                  i          =          1                ,        …        ⁢                                  ,                  N          UE                                    Equation        ⁢                                  ⁢                  (          3          )                    Note that, in a 2-stage PMI codebook system, Win=Win(1)×Win(2) associated with PMI#1 and PMI#2.
Precoding in this way is not optimal and it may be desirable to provide an improved or at least an alternative way of generating precoders.
It is to be clearly understood that mere reference herein to previous or existing systems, methods, models, processes, procedures, practices, publications or other information, or to any problems or issues, does not constitute an acknowledgement or admission that any of those things individually or in any combination formed part of the common general knowledge of those skilled in the field, or that they are admissible prior art.